The human eye is a remarkably effective organ adaptable to a wide range of ambient light levels which can be functionally enhanced or improved by a host of optical instruments. For instance, visual function can be extended beyond the visible spectrum into the infrared portion of the spectrum by using night vision goggles and forward looking infrared devices (e.g., FLIRs) which are applicable to industrial applications, but particularly suited for military applications.
Night vision goggles (NVGs) are a helmet mounted binocular image intensifier. The NVG provides the user with a 30 to 40 degree intensified field of view with an enhanced acuity at low ambient light levels (quarter moon down through starlight). The latest version of NVGs, however, are fixed focus at optical infinity (&gt;20 ft.) which makes them incompatible for use with any object, such as a terrain board used for training purposes, closer than 20 ft. Terrain boards have typical width and height dimensions of 10 by 10 ft. and allow users to become familiar with the operation and capabilities of the NVGs in a safe benign environment. To effectively use the terrain boards, the objects must remain in focus over a distance of 5 to 15 ft. from the NVG which represents a problem to the latest version of NVGs. To overcome this problem a solution is required that will allow the NVGs to be focused at a near distance with a large depth of field (i.e., the distance over which an object remains in focus) for the training periods. The solution should be easy to use, inexpensive, and readily adaptable to different fixed focus optical systems.